Mold wash



United States Patent Ofiice 3,364,040 MOLD WASH George H. Criss,Pittsburgh, Pa., assignor to Harbison- Walker Refractories Company,Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed Feb.23, 1965, Ser. No. 434,675 4 Claims. (Cl. 10638.23)

ABSTRACT OF THE DISCLOSURE A sprayable mold wash comprising a nonplasticrefractory aggregate, pulverized crude fire clay, and a termentedpolysaccharide gum.

This invention relates to a wash for coating the interior surfaces ofmolds to be used in metallurgical casting. More particularly, thisinvention relates to a mold wash to be used for coating the interiorwalls of nonpermeable molds by spraying. This invention does not relateto the gas forming type mold washes which form a gaseous layer betweenthe mold wall and the casting.

The primary purpose of a mold wash or coating upon metallurgical moldsis to prevent the casting, that is, the solidified metal shape formed bypouring molten metal into the mold, from sticking to the mold. Also,mold washes prevent erosion of the surface of the mold, thus extendingmold life. Further, the cooling rate or the rate of heat loss from thecasting can be controlled by the use of mold washes thus eliminatingcertain casting defects known in the art as coring. Mold washes serveother purposes known to those familiar with the metal casting art.

The requirements for mold washes are several. Since a contemporary formof application of mold washes is spraying, a mold wash must be capableof being suspended in a slurry and forced through a spray nozzle withoutclogging the nozzle. It must not settle out of suspension or requireconstant agitation to keep in suspension. Furthermore, the mold must notgel to any appreciable extent, that is, it must not thicken and becomestiif. Since a certain amount of gel is, however, required in all moldwashes so the solids will remain in suspension, they :4

are usually thixotropic, that is, when slightly agitated they are fluid.

The foregoing properties are required of a mold wash that is to besprayed. Other necessary properties are: adherence to the mold wallsespecially when sprayed on surfaces around 450 to 500 F., chemicallyinactivity with respect to the mold wall and the casting, and resistanceto spalling or cracking under rapid thermal change.

Prior mold washes have had variousof the desired properties discussedabove to some degree. The reactivity of many prior washes has given thema certain degree of instability: They have tended to irreversibly set upor gel when allowed to stand for any appreciable period of time. Theyhave tended to have solid constituents settle out.

Generally, most mold washes are comprised of two components; namely, anaggregate and a suspending agent or binder. The aggregate is chosen sothat the mold wash has the desired chemical and thermal shock resistantproperties. The binder is chosen so that the mold wash has 3,364,04aPatented Jan. 16, 1968 requisite suspending and adhering properties.Very common binders have been alkaline metal silicates.

It is, therefore, an object of this invention to provide a sprayablemold wash characterized by good stability over extended time periods,for example, over 8 hours.

It is another object of this invention to provide a mold wash of goodthermal shock resistance, not wettable by normal casting metals, whicheasily is sprayed, and which is characterized by good adherence to moldwalls, especially at elevated temperatures, for example, temperaturesbetween 450 to 500 F.

The preferred embodiment of this invention, and the best mode now knownfor practicing it, is as follows. A calcined fireclay is crushed andsized to have the following screen analysis:

Screen analysis (by weight) Percent Held on mesh 5 Pass 65 mesh held onmesh 10 Pass 100 mesh held on mesh 10 Pass 150 mesh held on 200 mesh 10Pass 200 mesh held on 325 mesh 10 Pass 325 mesh 55 The mold Washincluding the sized calcined fireclay is batched as follows:

Batch Percent Pulverized Crude Fireclay (-325 mesh) 5 FermentedPolysaccharide Gum 0.1 Sized Calcined Fireclay By difierence The milledor pulverized crude fireclay can be any plastic fireclay which has beenpulverized, for example, in a hammer mill. By crude it is meant that thefireclay has not been calcined. Plastic clays are characterized by theirgreat affinity for water and fine particle makeup. These propertiespromote the formation of colloids on addition of water which areresponsible for the plastic and suspending properties of such clays.

The fermented polysaccharide is of the type formed by reacting orallowing the action of the bacteria X anthamonas campestris on naturalgums, for example, of the type disclosed in US. Patent 3,067,038 andsold under the trade name of Kelzan (of the Ke-lco Co., Clark, N.J.).These gums are known in the art as Xanthomonas hydrophilic colloids.

An exemplary chemical analysis (on an oxide basis) of the crude plasticfireclay and calcined fireclay used in the preferred embodiment are asfollows:

Passing 325 mesh This invention may also be practiced by selecting anaggregate from the group comprised of calcined bauxite,

metal surface preheated to 500 F. The results of these tests arecontained in the following table.

TABLE I Examples III Aiter 8 hours. I Gelling After 1 hour Alter 8 hoursSprayability:

After 1 hour After 8 hours Adherence on Hot Metal Plate: MSG-500 E)" 1Not tested because of gellation.

Screen analysis (by weight) Held on 65 mesh Up to 10%. Pass 65 mesh heldon 100 mesh Pass 100 mesh held on 150 mesh Pass 100 mesh held on 200mesh Pass 200 mesh held on 325 mesh By difference.

Broadly, the mold wash, including the sized aggregate, is batched asfollows:

Batch (by weight) Pulverized Crude Fireclay (or functionally equivalentpl a s t i c refractory The following examples are offered, not by wayof limitation, but to more clearly teach the invention to those skilledin the art.

Mold washes were prepared by adding various binders, dispersing agentsand gums, as given in Table I, to a calcined firecl'ay aggregate. Theaggregate was sized so that no more than 5% was held on 65 mesh and atleast 55% passed 325 mesh. Suflicient water was added in order to bringthe wash to a spraying or painting consistency. Two tests were thenperformed on the resulting mold washes. First, a sample of each of themixes was placed in a 100 milliliter graduate to observe settling andgelling over an 8 hour period. Second, the mixes were sprayed onto aNone Excessive Excessive Excessive Slight None Excessive ExcessiveExcessive Slight None Viscous Viscous Gelled G elled None Excellent PoorPoor P001 Poor Excellent Excellent By excessive settling after 1 hour Imean there is more than one milliliter of clear liquid on the top of themilliliter graduate. By excessive settling after 8 hours I mean there ismore than 3 milliliters of clear liquid on top of the 100 millilitergraduate.

Examples I through IV demonstrate that certain water soluble polymers inthe known class polysaccharides do not fulfill the requirements of thisinvention (see Example V). Nor do the cellulose derivative gums sufiice(see Examples III and IV). Only the polysaccharide gums of the fermentedtype prevented settling and gelling of the mold washes.

Additional tests were run to determine the separate roles of thefermented polysaccharide gums and the pulverized crude clay. Examples VIand VII were prepared and tested in the same manner as Examples Ithrough V reported in Table I. The batch compositions and the testresults of these examples are given as follows:

that the pulverized crude clay or the polysaccharide gum of thefermented type, alone, do not provide the proper ties desired. It is thecombination of the two which is the essence of my invention.

Examples VIII and IX, given in Table III, were prepared in the samemanner as Examples I through VII. Table III establishes that the amountof polysaccharide gum of the fermented type added to the mix iscritical. When used in excess of 0.2% the mix is viscous and containslumps which do not dissolve. Such a viscous gel, and

the lumps, would be detrimental to sprayability. The mold wash wouldclog the nozzle through which the Wash is sprayed.

TABLE III Examples I VIII IX Batch (percent):

Calcined Fireclay 94. 9 94. 8 94. 7

Pulverized Crude Clay 5. 5.0 5. 0

Fermented Polysaccharide Gum. 0. 1 0. 2 0.3 Water Added 5o 49 47Settling:

After 1 hour None None None After 8 hours N one N one None Gelling:

After 1 hour None Viscous Viscous After 8 hours None N one None Allscreen analyses are based on the Tyler standard screen scale.

Having thus described the invention in detail and with sutficientparticularity, as to enable those skilled in the art to practice it,what -I desire to have protected by Letters Patent, is set forth in thefollowing claims:

I claim:

1. A method of coating nonpermeable metallurgical molds comprising thesteps of: preparing a refractory mold wash by mixing a batch ofmaterials consisting essentially of about 0.05 to 0.2%, by weight, of aXanthomonas hydrophylic colloid prepared by the use of the bacteriumXanlhomonas campestris, about 2 to 10%, by

weight, of pulverized ground fircclay and the remainder nonplasticrefractory aggregate sized so that no more than about 10% is held onabout 65 mesh and about to 90% passes 325 mesh, adding sufficient waterto the batch to bring it to spraying consistency, and emp'lacing thewash on the walls of the metallurgical molds over a period as long as 8hours after mixing.

2. The method according to claim 1 in which the nonplastic refractoryaggregate is sized so that no more than about 5% is held on 65 mesh andno less than about 55% passes 325 mesh.

3. The method according to claim 1 in which about 0.1% Xanthomonashydrophylic colloid. is added to the batch.

4. The method according to claim 1 in which the amount of water added tothe batch is about References Cited UNITED STATES PATENTS 3,110,94411/1963 Phelps 10638.22 3,146,113 8/1964 Middleton et al. l06-38.233,230,056 1/1966 Arant 106-38.22 X 3,232,929 2/1966 McNeeley et al.260-209 3,236,831 3/1966 Schweiger 260209 3,243,397 3/1966 Herkimer etal. l06-38.22 3,251,768 5/1966 Walker 260209 X ALEXANDER H. BRODMERKEL,Primary Examiner. L. B. HAYES, Assistant Examiner.

